This invention relates to a strip line coupling circuit having a strong coupling characteristic over a wide band.
Heretofore, a variety of microwave strip line coupling circuits have been proposed and used. For example, there is known a strip line coupling circuit, as shown in FIG. 1, which is provided with three distributed-coupling type directional couplers 1, 2 and 3 each as long as 1/4 of the wavelength of a microwave used at the center frequency, formed of two microwave strip lines. In order to form a 3 dB coupling circuit with such a good frequency characteristic as shown in FIG. 2 from that strip line coupling circuit, however, it is required that the degree of coupling of the directional coupler 2, which is supposed to have the highest one, should be set at 1.6 to 1.7 dB. It is very difficult to provide such strong coupling by bringing both those strip line conductors close to each other.